The invention relates to cooling of a mold used in a blow-molding process, and more particularly to cooling a mold or sections of a mold by recovering energy from the compressed air or gas used to operate a molding machine and to shape the containers in the mold.
In a typical blow-molding process employed in the manufacture of plastic containers, such as PET (polyethylene terephthalate) bottles, the plastic starting material is heated to about 95° C., which is 20° C. above its glass transition temperature. The supplied heat softens the plastic starting material so it can be stretched to and shaped to fill the mold. Compressed air at a pressure of about 30 bar and a temperature between about 20° C. and 30° C. is blown in the interior of a preform of the container, urging the container against the walls of the mold. The container hereby takes on the shape of the mold cavity.
Before the blow-molded container is removed from the mold, the mold is cooled to below the glass transition temperature of the plastic material, i.e., below about 70° C. for PET. In current molding machines, the mold is cooled by flowing chilled water at about 12° C. through cooling channels arranged in or on the mold. The water is chilled in a closed-loop refrigeration system and pumped through insulated pipes systems to the blow mold, where it flows through the cooling channels. During the molding process, the water temperature rises by about 2° C. The water is then returned from the mold to the refrigeration system to remove heat.
Water-cooled systems are subject to scale buildup and corrosion, are expensive to maintain and require a supply of external energy to chill the water, while the energy contained in the compressed gas used in the blow-molding process is wasted, as the compressed gas is simply vented to the ambient environment.
It would therefore be desirable to provide a system and method for cooling a blow-molding machine using less energy.